Oren Solomianik's Blog

EC2

Providing your web application with a mail service that works flawlessly is probably essential for your business. You need to send activation emails to users, password reset emails, newsletters and probably a whole bunch of other emails that have to do with interactions with your application.

When there were only physical servers and static IP addresses, everything worked perfectly. But now, when your application is in the cloud, setting up a working mail server next to your application is ridiculously impossible. If your application is successful and you would like to send emails to your millions of satisfied users, your options come down to:

Use a physical hosted server.

Use a 3rd party email service.

Set up a mail server in the cloud and compromise on some/most being marked as spam.

For us cloud oriented developers, option 1 is as useful as somebody suggesting you’d use a cassette tape recorder to put your favorite songs on. It’s old, unreliable, can’t scale. Option 2 is very costly if your business is successful, and most of theseservices don’t deal with the amount of mails you need to send if you have a large scale user base. Option 3 will make your email communication efforts with your users almost non-existent, which means you can’t afford it as well. So your only option is to compromise somewhere.

Why is sending email from the cloud so difficult?

In order for your mail server to operate successfully and be trusted by mail services around the world, you need to abide by the following rules:

Rule #1 is easily implemented in any mail server configuration, and there are also a number of onlinetools to test if you’re an open relay or not. Option #2 is also pretty easy to implement, assuming you control your DNS zone files and know your way around it.

The problem of mail on the cloud begins with rules #3 and #4. A PTR record, which is a reverse DNS entry, must be present and correct for your mail server to not be considered spammy. If your mail server is at 1.2.3.4 and is called mail.example.com, the PTR query for 1.2.3.4 (well, for 4.3.2.1.in-addr.arpa) must return mail.example.com. The PTR record can only be changed by the owner of the IP address, or by a delegation of his authority to you. Amazon Web Services do not let you control PTR records, so there goes the option for a mail server on EC2.

Other clouds let you control the PTR records for the IP addresses they assigned to you. But they fail on Rule #4. While your specific IP address might not be blacklisted in RBLs, the entire block that it belongs to might be blacklisted, because these IP addresses are assigned dynamically and therefore are always suspected as spammy by these lists. This is the case with Rackspace Cloud for example, and is the only thing left to be solved before you can run a mail server there. And although they’re trying to get their address block de-listed, this problem still persists.

Other clouds I’ve examined in this space are GoGrid and Joyent. GoGrid want you to fill up a questionnaire, and only then they open up port 25 for you. This sounds absurd, and against all the on-demand nature of the cloud (and I also personally don’t trust ServePath, the company that operates GoGrid). Joyent’s offering seem to disregard the option of hosting a mail server with them, and I couldn’t get their response on this matter.

So unless Rackspace Cloud solve their IP block blacklisting problem, or AWS offer a PTR setting option (plus no blacklisting as well), we’re left with the need to compromise.

The only feasible solution right now — seems like it’s back to physical hosting.

If you would like to cron snapshots of any attached volume to an instance, you can use the following script. It uses the EC2 command line tools to see what volumes are currently attached to this instance, and takes a snapshot. Make sure to replace all the variables on the top of the script to match your own.

If you’re wondering why $MOUNTPOINT is important (it’s not used here after all), it’s because you might want to freeze your filesystem if it’s XFS, so you could safely take a snapshot of a MySQL database for example. So you could easily wrap the snapshot create command with this:

And if you are indeed using this script to snapshot a volume with MySQL on it, you need also to flush tables with read lock, and gather information on master and slave positions. For this task you can use Eric Hammond‘s script, and incorporate it to the cron script. (You can read more about MySQL and XFS on EC2 on the AWS site).

Cloud computing has brought along the promise of easy-to-scale-and-yet-affordable computer clusters. There are various clouds out there that provide Infrastructure as a Service, such as Amazon EC2, Google App Engine, Mosso, and the newcomer Force.com Sites to name a few. I personally have experience as a developer only with Amazon EC2, and I am a devoted fan and user of the entire AWS stack. Nonetheless, I believe that what I have to say here is relevant to all other platforms.

While the cloud and IaaS model have indeed many significant advantages over traditional physical hosting, there is one major annoyance still to overcome in this space, and that is: your virtual host is still connected to a physical machine. And that machine is non-redundant, it doesn’t have any hot backup, and there’s no way to transparently and hassle-free fail over from it once its malfunctioning. And this is why, from time to time I get this email from Amazon:

Hello,

We have noticed that one or more of your instances are running on a host degraded due to hardware failure.

i-XXXXXXXX

The host needs to undergo maintenance and will be taken down at XX:XX GMT on XXXX-XX-XX. Your instances will be terminated at this point.

The risk of your instances failing is increased at this point. We cannot determine the health of any applications running on the instances. We recommend that you launch replacement instances and start migrating to them.

Feel free to terminate the instances with the ec2-terminate-instance API when you are done with them.

Let us know if you have any questions.

Sincerely,

The Amazon EC2 Team

At this stage, this is one of the greatest shortcomings of EC2 from my point of view. As a customer of EC2, I don’t want to care if a host has hardware failure. Why can’t my instance just be mirrored somewhere else, consistent hot-backup style, and upon failure of host hardware be transparently switched to the backup host? I don’t care paying the extra buck for this service.

In my vision, in a true IaaS cloud there is no connection between the virtual machine and the physical host. The virtual machine is truly floating in the cloud, unbound to the physical realm by means of some consistent mirroring across physical hosts.

And you might be thinking “you can implement this on your own on the existing infrastucture that EC2 offers”, and “you should be prepared for any instance going poof”. And you are correct, at the current offering of EC2, this is the case. You always have to be prepared for an instance failure (in the last month, I had 2 physical hosts failure out of about 20, that’s about a monthly 10% (!!) ), and you always have to build your architecture so that a single host failure can fail over gracefully. But were my vision a reality, I wouldn’t have to worry about these things, and wouldn’t have to spend time and money on the overhead that they incur.

I am not certain that this is the situation in the other clouds, but if it is not, it might come with the price of less flexibility, which is a major part of EC2 on which I am not willing to give up. If that flexibility can be maintained, I would love to see my vision become a reality on EC2.

I couldn’t find any info out there comparing network latency across EC2 Availability Zones and inside any single Availability Zone. So I took 6 instances (2 on each US zone), ran some test using a simple ping, and measured 10 Round Trip Times (RTT). Here are the results.

Single Availablity Zone Latency

Availability Zone

Minimum RTT

Maximum RTT

Average RTT

us-east-1a

0.215ms

0.348ms

0.263ms

us-east-1b

0.200ms

0.327ms

0.259ms

us-east-1c

0.342ms

0.556ms

0.410ms

It seems that at the time of my testing, zone us-east-1c had the worst RTT between 2 instances in it, almost twice as slow as the other 2 zones.

Cross Availablity Zone Latency

Availability Zones

Minimum RTT

Maximum RTT

Average RTT

Between us-east-1a and us-east-1b

0.885ms

1.110ms

0.937ms

Between us-east-1a and us-east-1c

0.937ms

1.080ms

1.031ms

Between us-east-1b and us-east-1c

1.060ms

1.250ms

1.126ms

It’s worth noting that in cross availability zones traffic, the first ping was usually off the chart, so I disregarded it. For example, it could be anywhere between 300ms to 400ms, and the the rest would fall down to ~0.300. Probably some lazy routing techniques by Amazon’s routers.

Conclusions

Zones are created different! — At least at the time of the testing, if you have a cluster on us-east-1b it performs almost twice as fast with regards to RTT between machines than a cluster on us-east-1c.

Cross Availability Zones latency can be 6 times higher than inner zone latency. For a network intensive application, better keep your instances crowded in the same zone.

I should probably also make a throughput comparison between and across Availability Zones. I promise to share if I get to test it.

EBS snapshots are a very powerful feature of Amazon EC2. An EBS volume is readily available, elastic block storage device that can be attached, detached and re-attached to any instance in its availability zone. There are numerous advantages to using EBS over the local block storage devices of an instance, and one of the most important of them is the ability to take a snapshot of the data on the volume.

Since snapshots are incremental by nature, after an initial snapshot of a volume, the following snapshots are quick and easy. Moreover, snapshots are always processed by Amazon’s processing power and not by the cpu of your instance, and are stored redundantly on S3. This is why using these snapshots in your backup methodology is a great idea (provided that you freeze/unfreeze your filesystem during the snapshot call, using LVM or XFS for example).

But, and this is a really annoying but – snapshots are “easy come hard to go”. They are so convenient to use and so reliable, that it’s natural to use a cronned script to make a daily, or hell — hourly! — backup of your volume. But then, those snapshots keep piling up, and the only way to delete a snapshot is to call a single API call for a specific snapshot.If you have 5 volumes you back up hourly, you reach the 500 snapshots limit withing 4.5 days. Not very reliable now, huh?

I have been searching for a while for an option to bulk delete snapshots. The EC2 API is missing this feature, and the excellent ElasticFox add-on is not compensating. You just can’t bulk delete snapshots.

That is, until now :). I asked in the AWS Forum if there is anything that can be done about this problem. They replied it’s a good idea, but if I really wanted it to be implemented quickly, I should build my own solution using the API. So I took the offer, and came up with a PHP command line tool that tries to emulate a “ec2-delete-old-snapshots” command, until one is added to the API.